Shifting light

Have you ever pondered the movement of a star
A planet or a galaxy up in the night sky
Have you ever wondered why the sound of a siren
Is constantly changing as it passes you by

It's good to have many questions
About the world that you face
That's how all of human kind's
Discoveries are made

The sound of that siren and light of that star
Both are ever changing, it depends on where you are
In relation to the source of sound or light alike
The frequency that you receive very well might

Cause sound to become lower in pitch
Or light to red-shift
This change in observed frequency
Found by doppler, the physicist

Just as the sounds of sirens change
As they come and go
The light from other galaxies
Helps us come to know

The universe is vast in size
Growing all the time
As shown by red-shifting light
That's come to meet our eyes